Diagnosis

• The diagnostic criteria change in the course of treatment.
  • During first aid, pain is the primary symptom in younger patients. Presentation in the elderly is often atypical.
  • When thrombolytic therapy is considered, an ST elevation on the ECG or a recent left bundle branch block (LBBB) should be taken into account. See the Finnish Medical Society Duodecim guideline "Thrombolytic Therapy in Acute Myocardial Infarction."
  • In addition to pain and ECG findings, myocardial enzyme levels are needed for definite clinical diagnosis.
• For differential diagnosis of chest pain, see the National Guideline Clearinghouse (NGC) summary of the Finnish Medical Society Duodecim guideline Differential Diagnosis of Chest Pain.
• The pain in MI lasts over 20 minutes and is localized widely in the retrosternal area, transfers to the arms, back, neck, or lower jaw. The pain is squeezing and is experienced as tightness, heaviness, and pressure or pressing. Breathing or changing posture does not influence the intensity of pain. The pain is usually severe and consistent. It may be localized in the upper abdomen, in which case, if nausea and vomiting are also present, it simulates acute abdominal disease. The patient is often pale, in a cold sweat, and anxious.
• MI may also present as acute pulmonary oedema, unconsciousness, or sudden death.
• Thrombolytic therapy is indicated
  • if the pain has lasted less than 6 to 12 (24) hours and there is at least a 2-mm elevation in the ST segment in at least two chest leads, or
  • a 1-mm elevation of ST in at least two leads in the extremities, or
  • a reciprocal ST depression in V1–V4, or
  • a recent left bundle branch block
• The contraindications for thrombolytic therapy must always be considered. See the Finnish Medical Society Duodecim guideline "Thrombolytic Therapy in Acute Myocardial Infarction."
• In clinical investigation, remember that the ECG and myocardial markers change with the course of the disease: first there is an ST elevation, after that development of the Q-wave, and finally T-wave inversion. Complications must also be recognized. In a T-wave infarction (non-Q-wave infarction), no classical Q waves are present, but the diagnosis is based on an increase of myocardial enzymes, chest pain, or ST-T changes. Classical Q-wave changes, ST elevations, and T inversions may be caused by various other diseases, which should be remembered in the differential diagnosis. An old infarction, bundle branch block, and early repolarization make the diagnosis difficult, in which case the change in ECG is important and an old ECG recording valuable. When added to other criteria, "minor" signs of infarction are also important.
• The European Society of Cardiology and the American College of Cardiology have agreed on a new definition of MI ("Myocardial infarction redefined," 2000):
  • Typical increase in the concentration of serum cardiac troponins or creatine kinase isoenzyme containing M and B subunits (CK-MB) associated with at least one of the following:
    • symptoms of cardiac ischaemia
    • recent pathological Q waves in the ECG
    • ischaemic ST segment changes in the ECG
    • coronary artery revascularization